TENS of millions of pounds could be heading to Scotland once the UK Government decides on the new multi-billion pound expenditure on airport expansion in south-east England, which is expected to be announced soon.

Sources in the SNP have stressed that if, as expected, Theresa May’s government gives the green light to airport expansion as part of its renewed emphasis on building up Britain’s infrastructure, then Scotland should not lose out but receive a proportionate share of extra public spending, usually around nine per cent, through the so-called Barnett Formula system.

Any additional money will be up to the administration in Edinburgh to decide how, where and when to spend it.

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A decision on the fraught issue of where to build a new runway in south-east England – either a third runway at Heathrow or a second at Gatwick – is expected soon; certainly by the end of the month and even possibly this week.

David Cameron, the former Prime Minister, famously declared that there would be no third runway at Heathrow “no ifs no buts,” but a report he commissioned in 2015 came out firmly in support of the Heathrow option, estimating the overall cost at £18.6 billion.

The decision has been delayed while Whitehall assesses the environmental impact; last week, independent research claimed Heathrow could build a new runway without breaching pollution laws.

The delay in a decision has also seen a change, possibly significantly, in personnel at No 10.

A senior Nationalist source said: “Inevitably, if there is new spending to provide additional infrastructure, then, as far as Scotland is concerned, that would have to be subject to the consequentials effect of the Barnett Formula.”

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While the Scottish Government has said it is remaining impartial on the airport expansion decision, SNP MPs are believed to favour expanding Heathrow. Their role at Westminster, where the Prime Minister has a working majority of just 16, could prove crucial given there are divisions within Tory ranks on the issue.

Thus far, Edinburgh is the only major airport in Scotland to offer its support to the Gatwick expansion; Glasgow, Aberdeen and the Highlands and Islands all favour the Heathrow option.

Whichever way the UK Government goes on airport expansion, there will be a major political row.

The aftermath will not only likely include legal challenges by local authorities, a swathe of protests but also heated debate at Westminster and beyond. A vote by MPs will follow and, if it is not a free one, could well see some ministers resigning from the Government; Justine Greening, the education secretary, who is MP for Putney under the Heathrow flight path, has been a vociferous opponent to a third runway.

Boris Johnson, the former London mayor and now foreign secretary, is also strongly opposed to the Heathrow option. Last year, Philip Hammond, the new chancellor whose constituency is in Surrey, came out in favour of expanding Gatwick.

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There has been talk of No 10 devising a way to enable ministers to be “unavoidably away” on the day of the Commons vote to avoid the embarrassment of cabinet resignations.

Another opponent, Zac Goldsmith, the Tory MP for Richmond Park, has made clear he will spark a by-election if the Government opts for the Heathrow option.

A poll of MPs last month showed two-thirds of them backed the west London option.

Meantime, Gatwick Airport has insisted it would remain "ready to deliver" a second runway even if the UK Government rejected its proposal in favour of expanding Heathrow.

If it were selected for a third runway, it would expect to receive final planning permission by around 2020.

The airport wants to add 25,000 more flights to its existing annual limit of 480,000 between 2021 and the opening of a third runway in 2025 to provide a "Brexit boost".

Gatwick has said it is 80 per cent confident that it could deliver an operational second runway by 2025.